1. Field of the Invention
This invention relates to optical distortion calibration for electro-optical sensors.
2. Description of the Related Art
All imaging systems have some amount of distortion attributable to their optics. Non-linear distortion causes a fixed angular displacement between points in image space to appear to change as the points move across the image. In other words, the observed line-of-sight (LOS) positions are warped. Common types of distortion include pincushion or barrel distortion. Some applications, notably precision stadiometry, require that camera distortions be precisely calibrated, so that measurements may be post-compensated. Calibration is markedly more difficult in systems where the required precision or other conditions, such as operation in cryo vacuum conditions, make it impractical to project precision collimated patterns that fill the sensor's entire field of view (FOV) necessitating that a smaller pattern be scanned across the FOV.
The current approach used to calibrate electro-optic (EO) sensors in a cryo vacuum chamber is time-consuming, expensive and limited in accuracy. A Theodolite is placed looking through a window in the cryo chamber, in place of the sensor and sensor optics. A single point target is projected through a collimator and moved in discrete steps across the FOV using a folding mirror. The mirror must stop at each point to allow the Theodolite to observe the actual position of the target in response to a command issued by a mirror controller. The mirror controller is than calibrated by computing a mirror transformation that converts the observed mirror positions to truth. The Theodolite and window are removed and the EO sensor and optics are placed in the test chamber. The mirror is moved to sweep the target across the FOV but again must stop at each point so that the mirror readouts can be synchronized to each image. The target position in each image is also measured. The mirror transformation is applied to the mirror position to remove that source of error and provide a calibrated line-of-sight truth position for each mirror position. The distortion correction function is calculated, generally as a 2nd order polynomial fit, to map the measured FOV position of the target in each frame to the calibrated line-of-sight truth position for each corresponding mirror position. The fit provides the coefficients required to post-compensate sensed images due to the non-linear distortion induced by the sensor optics. The steps of calibrating the mirror controller and having to stop at each mirror position to observe the target position are the primary limitations on cost, calibration time and accuracy.